Method of characterizing a co2 plume in a geological storage aquifer

ABSTRACT

A method of monitoring a CO 2  geological storage site by locating the CO 2  plume from 4D seismic data is disclosed. A stratigraphic inversion of the seismic data is performed in order to obtain the P and S impedances, before and after CO 2  injection. A density variation cube and an incompressibility modulus variation cube are constructed from the seismic impedances. The free CO 2  plume is located within the subsoil by identifying, within the cubes cells where the density variation is negative and where the incompressibility modulus variation is negative, which are of an absolute value greater than a given positive threshold.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the technical sphere of geological storage ofgreenhouse gas such as carbon dioxide (CO₂), and more particularly tothe monitoring of geological storage sites for such gases.

2. Description of the Prior Art

The scenarios established by the IPCC (Intergovernmental Panel onClimate Change) show that the CO₂ concentration in the atmosphere, inthe absence of any corrective measure, will evolve from a currentconcentration of 360 ppm to more than 1000 ppm by the end of theXXI^(st) century with significant consequences on the climate change.Capture of the CO₂ emissions from high volume sources (for example,thermal power plants), transportation of this CO₂ and storage thereof insuitable underground formations is one of the solutions available forreducing greenhouse gas emissions. CO₂ geological storage pilot projectsexist already, but continuing the deployment of this technology requireshigh-quality technologies in order to meet the requirements of theregulations that are being set up and to meet public expectations.

Deep saline aquifers have the highest potential for CO₂ geologicalstorage among all the geological formations being considered regardingtheir geographical distribution and their theoretical storage capacity.

The volume of the CO₂ injected in an underground geological formation iseasily known by measuring the gas flow rate at the wellhead. However,the fate of the CO₂ once injected is much more difficult to control:since CO₂ can migrate vertically out of the storage formation (to moresuperficial geological layers or even to the surface) or laterally intothe host formation in non-initially planned zones. Furthermore, the CO₂can undergo physico-chemical changes over time, likely to cause it totake different forms, among which are free form (gaseous orsupercritical), dissolved form, in brine, or a mineralized form forexample.

Thus, monitoring as completely as possible the fate of CO₂ has to becarried out in order to meet the regulations in force, and to helptowards societal acceptance of this technology. This complete monitoringmust involve detecting leakage out of the geological storage formationand quantifying such leaks, as well as a volume and/or mass balance ofthe CO₂ in place in the geological storage formation.

The following documents reflect the state of the art:

-   Arts, R. et al., 2002. Estimation of the Mass of Injected CO₂ at    Sleipner Using Time Lapse Seismic-Data, Expanded Abstracts of the    64th EAGE, Florence 2002, paper H016.-   Calvert, R., 2005, Insights and Methods for. 4D Reservoir Monitoring    and Characterization, SEG/EAGE DISC (Distinguished Lecture Course),    2005.-   Chadwick, R. A., Arts, R. et O. Eiken, 2005, 4D Siesmic    Quantification of a Growing CO₂ Plume at Sleipner, North Sea, In:    DORE', A. G. & VINING, B. A. (eds) Petroleum Geology: North-West    Europe and Global Perspectives—Proceedings of the 6^(th) Petroleum    Geology Conference, 1385-1399.-   Bourbié, T., Coussy, O. et B. Zinszner, 1987, Acoustics of Porous    Media, Editions Technip, Paris.-   Rasolofosaon P., Zinszner B., 2004, Laboratory Petroacoustics for    Seismic Monitoring Feasibility Study. The Leading Edge, v. 23; no.    3, p. 252-258.-   Rasolofosaon, P. N. J. and Zinszner, B. E., 2007. The Unreasonable    Success of Gassmann's Theory . . . . Revisited, Journal of seismic    Exploration, Volume 16, Number 2-4, 281-301.-   Zinszner, B. et F. M. Pellerin, 2007, A Geoscientist's Guide to    Petrophysics, Editions Technip, Paris.

Many techniques have been developed by industrialists in order tomonitor the evolution of the fluids produced or injected in a porousmedium. Among these techniques, the repetitive seismic technology,referred to as 4D seismic technology, is used in the (petroleum orenvironmental) industry. Such a technique carries out various seismicsurveys, at different times (the surveys are generally carried out atone year intervals at least). Thus, specialists can follow the evolutionof the fluids of the reservoir under production or of the geologicalstorage site (Calvert, 2005, for example).

The seismic data (velocities), which are estimated from the acquireddata, allow obtaining the elastic properties of the fluids produced orinjected by means of a theoretical model, generally of poroelastic type(Biot-Gassmann) (for example, Bourbié et al., 1987, Rasolofosaon andZinszner, 2004 and 2007).

All these techniques have been exploited in the environmental sphere toestimate, from seismic data, the total volume and the total mass of CO₂in place in the subsoil.

For example, Arts et al. (2002) exploit the measurements of the delaytaken by the seismic wave to travel through the subsoil layers invadedby the CO₂, in relation to a faster propagation in the brine-saturatedsubsoil, in order to locate the CO₂-invaded zone and to estimate thetotal volume of CO₂ in place. These authors use Gassmann's theoreticalmodel (for example, Rasolofosaon and Zinszner, 2004 and 2007).Furthermore, assuming that the mean density of the CO₂ under thereservoir conditions (pressure and temperature) are known, they canestimate the total mass of the CO₂ in place, which they compare more orless successfully with the mass of CO₂ really injected.

In a similar but more sophisticated approach, Chadwick et al. (2005)exploit not only the wave propagation time data, but also the amplitudesof the seismic waves. They obtain somewhat finer estimations of the CO₂distribution and of the total mass of the CO₂ in place than the previousauthors, without however reaching the three-dimensional distribution ofthe CO₂ saturations as provided by the invention.

The major drawbacks of the previous methods can be summed up in two mainpoints:

First, the previous analyses are essentially based on the analysis ofthe wave propagation times and amplitudes, and not on a completeinversion of the seismic data, with a really quantitative estimation, atany point of the subsoil, of the elastic parameters (impedances,incompressibility moduli, etc.). Now, by analyzing the times or theamplitudes, it is not possible to perform a true quantitative analysis.In fact, if for example the analysis of the propagation time variationsin the storage layer due to CO₂ injection is performed, an estimation ofthe overall velocity variation in the entire layer (and not local, ateach point) is obtained,

Second, all these methods are based on the use of an elastic model ofthe porous medium, whose robustness needs no further proof, but whosekey parameter estimation (drained incompressibility and shear modulinotably, and grain compressibilities to a lesser extent) still is aproblem (for example, Arts 2002 and Calvert et al. 2005).

SUMMARY OF THE INVENTION

The present invention provides a method based on seismic data analysisof quantifying the CO₂ in free form in a geological storage site.

The method of the invention allows not utilizing these theoreticalmodels while remaining close to the seismic data alone. The inventionuses seismic inversion results (impedances) and exploits the fact thatCO₂ injection into an aquifer, preferably a deep saline aquifer, is afirst drainage physical phenomenon (Zinszner and Pellerin, 2007).

The invention thus relates to a method of monitoring a CO₂ geologicalstorage site, from a first set of seismic data imaging a subsoil zoneand acquired before a CO₂ injection into an underground formation in thezone, and from a second set of seismic data imaging the subsoil zone andacquired after CO₂ injection. The method comprises locating the free CO₂plume formed after the injection by carrying out the following stages:

constructing by use of a stratigraphic inversion for each seismic dataset a P-wave seismic impedance cube and an S-wave seismic impedancecube;

constructing a density variation cube from the P-wave and S-wave seismicimpedance cubes before and after CO₂ injection wherein the densityvariation cube discretizes the subsoil zone into a set of cells;

constructing an incompressibility modulus variation cube from the P-waveand S-wave seismic impedance cubes before and after CO₂ injection,wherein the incompressibility modulus variation cube discretizes thesubsoil zone into a set of cells; and

locating the free CO₂ plume in the zone by identifying cells where thedensity variation is negative and where the incompressibility modulusvariation is negative, and of absolute value greater than a givenpositive threshold to locate the free CO₂ plume.

According to the invention, it is also possible to determine a volume ofthe free CO₂ plume by carrying out the following stages:

constructing a CO₂ saturation cube in the formation, from a densityrelative variation cube; and

determining the volume of the free CO₂ plume by adding all values of theCO₂ saturation cube, and by weighting the sum by a mean porosity of theformation and by a volume occupied by an elementary cell of the CO₂saturation cube.

A mass of the free CO₂ plume can also be determined by multiplying thevolume of the free CO₂ plume by a mean density of the CO₂.

The CO₂ saturation cube can be constructed by carrying out the followingstages:

determining an irreducible brine saturation S_(wi) for the formation;

determining a maximum absolute value for said density variation cube andcalculating a ratio by dividing the maximum absolute value by 1−S_(wi);

constructing a CO₂ saturation cube by multiplying each value of thedensity variation cube by the ratio; and

converting the CO₂ saturation cube to depth with a time-depth conversiontechnique.

According to the invention, the integrity of the storage site can bechecked by detecting a CO₂ leak through analysis of the location of thefree CO₂ plume. Remediation techniques can then be implemented to stopthe leak.

According to the invention, the storage site exploitation conditions canbe modified after detecting a transformation of free CO₂ by comparisonof either the volume of CO₂ with a volume of CO₂ injected, or of themass of CO₂ with a mass of CO₂ injected.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the method according to the inventionwill be clear from reading the description hereafter of embodimentsgiven by way of non-limitative examples, with reference to theaccompanying figures wherein:

FIG. 1 illustrates the stages of the method according to the invention;

FIG. 2 shows a horizontal section, at a set depth at the top of the CO₂plume, of the thresholded cube RVKS of the incompressibility modulusrelative variations for four values of the threshold;

FIG. 3 shows three-dimensional views of the complete CO₂ plume,intersected by a southwest-northeast (left) and southeast-northwest(right) oriented vertical plane;

FIG. 4 illustrates horizontal sections at the level of the top (left)and the base (right) of the CO₂ plume of the CO₂ saturation cube SC; and

FIGS. 5 and 6 illustrate the quality of the results provided by themethod by comparing the total volume (FIG. 5) of CO₂ under downholeconditions and the total mass (FIG. 6) of CO₂ injected as a function ofthe calendar time, as well as the estimations obtained from the methodaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of quantifying the CO₂ in freeform in a geological storage site, based on the analysis of repetitiveseismic data referred to as 4D seismic data.

The method characterizes the CO₂ plume by determining the location andthe geographical extension of the plume, as well as its volume and mass.

FIG. 1 illustrates the stages of the method according to the invention.It comprises three main stages:

1. Data acquisition and CO₂ injection

2. CO₂ plume locating by:

-   -   Determining the seismic impedances (or seismic wave velocities)    -   Constructing density and incompressibility variation cubes    -   Locating the CO₂ plume

3. Determining the volume and the mass of the CO₂ plume by:

-   -   Constructing a subsoil CO₂ saturation cube    -   Determining the volume and the mass of the CO₂ plume.

1. Data Acquisition and CO₂ Injection

A first set of 3D seismic data is first acquired, before CO₂ injection(AI) into the aquifer. This first set allows an image of subsoil zonecontaining the injection aquifer to be obtained. This set is denoted bySD^((AI)). These data make up a first seismic amplitude cube.

CO₂ is injected into the aquifer.

A second set of 3D seismic data is then acquired (after CO₂ injection(PI)). This second set allows an image of the same subsoil zone as theone imaged by the first data set to be obtained. This set is denoted bySD^((PI)). These data make up a second seismic amplitude cube.

As a result two images of a zone of the subsoil containing the injectionaquifer are available. As it is known regarding 4D seismic methods, theacquisition device for these seismic data must be substantially the sameso as to obtain comparable images.

It is also possible to acquire, with a view to a stratigraphic inversion(stage 2.1), well data denoted by WD. These data comprise density logs,as well as P and S wave velocity logs.

2. CO₂ Plume Locating

Locating the CO₂ plume in the subsoil is based on the knowledge of therelative variation of the incompressibility modulus and the relativevariation of the density of the geological formations of the subsoilinvaded by the CO₂.

The incompressibility modulus, denoted by K, of a rock is theproportionality coefficient between the isotropic confinement pressureexerted on a sample of the rock and the relative volume variation (orvolume deformation) of the sample resulting from this pressure.

Determination of these cubes of variation within the subsoil is basedonly on the seismic impedances directly obtained from the seismic data.

2.1 Determining the Seismic Impedances (or Seismic Wave Velocities)

Determination of the seismic impedances is based on a stratigraphicinversion of the 4D seismic data cubes. Such a technique is well known.

According to an embodiment, the two seismic amplitude cubes areconverted to seismic impedances (or seismic velocities) by a prestackstratigraphic inversion (ISAS). A stratigraphic inversion method isdescribed in the following documents for example:

-   T. Tonellot, D. Macé, V. Richard, 1999, Prestack Elastic Waveform    Inversion Using a priori Information, 69^(th) Ann. Internat. Mtg:    Soc. of Expl. Geophys., paper 0231, p. 800-804.-   Tonellot T., Macé D. and Richard V. [2001]. Joint Stratigraphic    Inversion of Angle-Limited Stacks. 71^(st) SEG Annual International    Meeting, Expanded Abstracts, 227-230.-   A. S Barnola, B. Andrieux, T. Tonellot et O. Voutay, 2003, Pre-stack    Stratigraphic Inversion and Attribute Analysis for Optimal Reservoir    Characterization, SEG Expanded Abstracts 22, 1493 (2003);    doi:10.1190/1.1817576.

Well data WD allow construction and calibration of an a priori modeloften used in stratigraphic inversion techniques.

The data obtained after inversion are the P-wave and S-wave impedancecubes, respectively denoted by IP^((AI)) and IS^((AI)) for thepre-injection data, and by IP^((PI)) and IS^((PI)) for thepost-injection data.

All the data used are three-dimensional data cubes. The first twodimensions are horizontal geographical directions x and y, and the“vertical” third dimension is the seismic recording time t. These cubesthus represent a discretization of the subsoil to a set of cells. Eachcell is associated with a value of a property as a function of the cubeso that the cells of the P-wave impedance cube contain a P-waveimpedance value.

For the same depth, the recording time of a set of seismic data acquiredat a given time is not the same as the recording time of a second set ofseismic data acquired subsequently, because the velocities ofpropagation of the seismic waves in the rock are modified by thesubstitution of the CO₂ for the brine. Different time scales areinvolved.

Thus, the time scales are different for the pre- and post-injectionimpedance data.

As it is well known, the interpretation of 4D seismic data involves astage of matching the propagation times of the two seismic data setsobtained before and after injection. Such a technique is commonlyreferred to as warping (W). An example is described in the followingdocument:

-   DeVault B., et al., 2007, “Prestack 9-C Joint Inversion for    Stratigraphic Prediction in the Williston Basin”. 77^(th) SEG Annual    International Meeting, Expanded Abstracts, 1039-1043.

Thus, the post-injection impedance data IP^((PI)) and IS^((PI)) aretemporally matched with the pre-injection data IP^((AI)) and IS^((AI))to allow quantitative comparison thereof. The output data are the P-waveand S-wave post-warping impedance seismic data which are respectivelydenoted by WIP^((PI)) and WIS^((PI)) and are expressed in the same timereference frame as the pre-injection impedance data.

2.2 Constructing Density and Incompressibility Variation Cubes

The impedance data are first transformed into cubes of the product ofincompressibility modulus K by density ρ of the subsoil. These cubes aredenoted by KR^((AI)) for the pre-injection cube and by KR^((PI)) for thepost-injection cube.

These transformations are performed using the following formulas:

$\begin{matrix}{{{KR}^{({AI})}\left( {x,y,t} \right)} = {\left( {{IP}^{({AI})}\left( {x,y,t} \right)} \right)^{2} - {\frac{4}{3}\left( {{IS}^{({AI})}\left( {x,y,t} \right)} \right)^{2}}}} & (1)\end{matrix}$

before injection, and

$\begin{matrix}{{{KR}^{({PI})}\left( {x,y,t} \right)} = {\left( {{WIP}^{({PI})}\left( {x,y,t} \right)} \right)^{2} - {\frac{4}{3}\left( {{WIS}^{({PI})}\left( {x,y,t} \right)} \right)^{2}}}} & (2)\end{matrix}$

after injection.

These equations are due to the fact that the velocities of the P waves,V_(p), and of the waves, V_(s), and the associated impedances I_(p) andI_(s) respect the following equations:

${{\rho \; V_{P}^{2}} = {K + {\frac{4}{3}\mu}}};$ ρ V_(S)² = μ;I_(P) = ρ V_(P) and I_(S) = ρ V_(S)

The following formulas are deduced therefrom:

Kρ=I _(P) ²−(4/3)I _(S) ² and μρ=I _(S) ²

Cubes KR(AI) and KR^((PI)) are then transformed into cubes of theproduct of the rigidity (or shear modulus μ) by density ρ of thesubsoil. These cubes are denoted by MR^((AI)) for the pre-injection cubeand by MR^((PI)) for the post-injection cube.

These transformations are performed using the following formulas:

MR ^((AI))(x,y,t)=(IS ^((AI))(x,y,t))² before injection, and  (3)

MR ^((PI))(x,y,t)=(WIS ^((PI))(x,y,t))² after injection.  (4)

The cubes of the relative variations of products K.ρ and μ.ρ, due to theinjection of CO₂, are respectively denoted by RVKR and RVMR. They arededuced from the following formulas:

$\begin{matrix}{{{{RVKR}\left( {x,y,t} \right)} = \frac{{{KR}^{({PI})}\left( {x,y,t} \right)} - {{KR}^{({AI})}\left( {x,y,t} \right)}}{{KR}^{({AI})}\left( {x,y,t} \right)}},{and}} & (5) \\{{{RVMR}\left( {x,y,t} \right)} = \frac{{{MR}^{({PI})}\left( {x,y,t} \right)} - {{MR}^{({AI})}\left( {x,y,t} \right)}}{{MR}^{({AI})}\left( {x,y,t} \right)}} & (6)\end{matrix}$

By disregarding the pressure effects and the variations of the shearmodulus μ due to the substitution of the CO₂ for the brine, the densityrelative variation, RVR, and incompressibility modulus relativevariation, RVK, cubes are obtained from the following formulas:

RVR(x,y,t)=RVMR(x,y,t), and  (7)

RVK(x,y,t)=RVKR(x,y,t)−RVMR(x,y,t)  (8)

2.3 Locating the CO₂ Plume

Since the CO₂ has a lower incompressibility modulus and density than thebrine, substituting the CO₂ for the brine tends to decrease the densityand the effective incompressibility modulus of the rock in the injectionzone. The subsoil zones, where density ρ decreases, are located bycalculating the filtered cube of the relative density variations whichare denoted by RVRF and defined by:

$\begin{matrix}{{{RVRF}\left( {x,y,t} \right)} = \left\{ \begin{matrix}1 & {{{if}\mspace{14mu} {{RVR}\left( {x,y,t} \right)}} < 0} \\0 & {{{if}\mspace{14mu} {{RVR}\left( {x,y,t} \right)}} > 0}\end{matrix} \right.} & (9)\end{matrix}$

Similarly, the subsoil zones where incompressibility modulus K decreasesare located by calculating the filtered cube of the relativeincompressibility modulus variations, denoted by RVKF and defined by:

$\begin{matrix}{{{RVKF}\left( {x,y,t} \right)} = \left\{ \begin{matrix}1 & {{{if}\mspace{14mu} {{RVK}\left( {x,y,t} \right)}} < 0} \\0 & {{{if}\mspace{14mu} {{RVK}\left( {x,y,t} \right)}} > 0}\end{matrix} \right.} & (10)\end{matrix}$

Furthermore, as a result of the many noise sources, essentially linkedwith the acquisition, this incompressibility modulus decrease isspatially coherent only above a certain incompressibility modulusrelative variation threshold ε. The sufficient decrease zones forsubsoil incompressibility modulus K are located by calculating thethresholded cube of the incompressibility modulus relative variations,denoted by RVKS and defined by:

$\begin{matrix}{{{RVKS}\left( {x,y,t} \right)} = \left\{ \begin{matrix}1 & {{{if}\mspace{14mu} {{{RVK}\left( {x,y,t} \right)}}} > ɛ} \\0 & {{{if}\mspace{14mu} {{{RVK}\left( {x,y,t} \right)}}} < ɛ}\end{matrix} \right.} & (11)\end{matrix}$

It can be noted that cube RVKS is simply obtained through thresholding(S) of cube RVK of the incompressibility modulus relative variations.

FIG. 2 shows a horizontal section, at fixed depth at the level of thetop of the CO₂ plume, of the thresholded cube RVKS of theincompressibility modulus relative variations for four values of thethreshold: ε=5%, 10%, 15% and 20%. The points of the cube associatedwith values above the threshold are shown in black.

The zones exhibiting both a density ρ decrease and a sufficientincompressibility modulus K decrease are then located by calculating thethresholded and filtered cube of the incompressibility modulus relativevariations, denoted by RVKSF, and defined by:

RVKSF(x,y,t)=RVRF(x,y,t)×RVKF(x,y,t)×RVKS(x,y,t)  (12)

Finally, cube RVR of the density relative variations is filtered by theprevious cube so as to obtain the thresholded and filtered cube of thedensity relative variations, denoted by RVRSF and defined by:

RVRSF(x,y,t)=RVKSF(x,y,t)×RVR(x,y,t)  (13)

This cube thus has a set of cells of value 1 if the cell contains CO₂and of value 0 if it does not, thus locating the geographical extensionof the CO₂ plume.

FIG. 3 illustrates a result. It shows three-dimensional views of thecomplete CO₂ plume intersected by a southwest-northeast (left) andsoutheast-northwest (right) oriented vertical plane. The cut plane isshown in black, and only the part of the plume located at the back ofthe cut plane is shown (in grey). Some connections between the variouslayers that make up the plume, which are made visible by seismictechniques, are shown by arrows and by letter C.

3. Determining the Volume and the Mass of the CO₂ Plume

According to the invention, a subsoil CO₂ saturation cube is determinedin order to determine the volume and the mass of the CO₂ plume.

3.1 Constructing a Subsoil CO₂ Saturation Cube

Determination of the saturation within the subsoil is based on theknowledge of the density relative variation RVRSF of the geologicalformations of the subsoil invaded by the CO₂.

In the zones of maximum density relative variation, the CO₂ injectionhas however not totally expelled the brine in place. Some brine remainstrapped and the brine saturation is close to the so-called irreduciblebrine saturation, denoted by S_(wi) (0≦S_(wi)≦1). This parameter, whichis known to petrophysicists, is a characteristic of the porous mediumand it is a new input parameter. The typical values of this parameter inthe very porous and very permeable media selected for CO₂ storage areoften well below 0.1 and they can be as low as 0.05. The correspondingmaximum CO₂ saturation denoted by S_(CO) ₂ ^((MAX)) is given by thefollowing formula:

S _(CO) ₂ ^((MAX))=1−S _(wi)  (14)

The minimum CO₂ saturation is zero and it corresponds to the zones notyet reached by the CO₂ plume. Besides, the relative variation of therock density due to the substitution of CO₂ for brine being proportionalto the CO₂ saturation, the CO₂ saturation cube, identified by SC, isobtained by the formula:

$\begin{matrix}{{{SC}\left( {x,y,t} \right)} = {{{RVRSF}\left( {x,y,t} \right)}\frac{S_{{CO}_{2}}^{({MAX})}}{{RVRSF}^{({MAX})}}}} & (15)\end{matrix}$

where RVRSF^((MAX)) designates the maximum value (in absolute value) ofthe density relative variations calculated in cube RVRSF. Thisparticular density variation value corresponds to the irreducible brinesaturation condition.

FIG. 4 illustrates horizontal sections at the level of the top (left)and of the base (right) of the CO₂ plume of the CO₂ saturation cube SC.The CO₂ saturation is identified by S_(CO2) on the scale.

According to an embodiment, the data are transformed to depth. Asmentioned above, in all the previous data, the third “vertical”dimension is the seismic recording time t. In the next stage, timevariable t is transformed to depth variable z. A known technique istherefore used which is time-depth conversion (TDC).

Time-depth conversion (TDC) can be carried out on all the previous dataand, in particular, the CO₂ saturation cube SC. Cube SC is thusconverted to a CO₂ saturation depth cube denoted by SCP.

3.2 Determining the Volume and the Mass of the CO₂ Plume

The data cubes have cells known as “voxels” (contraction of the terms“volumetric pixels”). These cells are of known dimensions Δx, Δy and Δz.In the horizontal directions, these dimensions Δx and Δy are known as“inter-traces”. They are acquisition data corresponding to the distancesbetween the successive recording points for the same shot pointrespectively in directions x and y. Vertical dimension Δz is the seismicdata recording sampling rate Δz converted to depth by time-depthconversion.

The total volume of CO₂, denoted by VTC, can be calculated by adding upall the values of the CO₂ saturation depth cube SCP, and by weightingthe result by the volume occupied by a cell and by the mean porosity φof the rock, i.e.:

$\begin{matrix}{{VTC} = {\varphi \times \Delta \; x \times \Delta \; y \times \Delta \; z{\sum\limits_{x,y,z}\; {{SCP}\left( {x,y,z} \right)}}}} & (16)\end{matrix}$

The mean porosity φ of the rock, defined by the ratio of the volume ofthe pores to the total volume of the rock, is measured or estimated fromlogs or measurements on samples taken in wells (for example, Calvert,2005). This quantity is a characteristic of the porous medium and it isa new input parameter. The thermodynamic conditions (fluid pressure andtemperature) at the level of the injection point are generally known.From these data, thermodynamicists can calculate the mean density ρ_(CO)₂ of the CO₂ from tables of physical constants or by means of referencessuch as the reference as follows:

-   Span, R., Wagner, W., 1996 “A New Equation of State for Carbon    Dioxide Covering the Fluid Region from the Triple-Point Temperature    to 100 K at Pressures up to 800 MPa”, J. Phys. Chem. Ref. Data, Vol.    25, No. 6, pp. 1509-1596.

By introducing this mean CO₂ density, ρ_(CO) ₂ , the total mass MTC ofthe CO₂ can be estimated using the following formula:

MTC=ρ _(CO) ₂ ×VTC  (17)

Results

FIGS. 5 and 6 illustrate the quality of the results provided by themethod according to the invention for estimating the volume and the massof CO₂ in place.

FIG. 5 illustrates the total volume (VC) of CO₂ under downholeconditions as a function of the calendar time (TC). The curvecorresponds to the injection data, the grey rectangle represents theestimation of the volume of free CO₂ provided by the method according tothe invention.

FIG. 6 illustrates the total mass (MC) of CO₂ under downhole conditionsas a function of the calendar time (TC). The curve corresponds to theinjection data, the grey rectangle represents the estimation of the massof free CO₂ provided by the method according to the invention.

It can be observed that the estimation meets the data, thus validatingthe method.

Application

The method according to the invention for characterizing in detail, andnot only globally, a CO₂ plume can be integrated in a method forinjecting CO₂ into an underground formation, notably by allowing betterdescription of the variation over time of the fluid distributions in thegeological CO₂ storage level.

The method allows partition of the geological CO₂ storage level, tolocate the various subsoil compartments and the connectivities betweenthese various compartments. This precise information allows location andestimation of the CO₂ volumes actually stored in the subsoil and tocontrol the integrity of the geological cap rocks.

During geological storage of an acid gas such as CO₂, it is necessary tocheck that the underground reservoir (porous medium) into which the gasis injected is fully tight. However, gas may escape from this naturalreservoir. In order to remedy such leaks, various techniques, referredto as “remediation” techniques, have been developed by operators.

After determining the geographical extension, the volume and the mass ofthe CO₂ plume, the integrity of the storage site can be checked.

In fact, just as the spatial resolution of the method allows locationwith precision the close connections between the various layers thatmake up the CO₂ plume (see C in FIG. 3), the method of the inventionallows not only fine specification in 3D of the lateral extension of theplume, but also to detect unambiguously any free CO₂ leak through thecap rock which is in the upper part of the storage zone. The highspatial resolution of the method in this precise case is a necessity.Detecting possible free CO₂ leaks allows these remediation methods to beimplemented to prevent such leaks.

Furthermore, it is essential to monitor the evolution of the injectedgas in order to determine whether storage is successful in the desiredplace, to evaluate the amount actually stored and to control thereservoir integrity by checking that there is no free CO₂ leak. Thisinformation also allows determination if other injection wells arenecessary.

More generally, our understanding of the storage mechanisms can beimproved since the free CO₂ does not represent all of the CO₂ injectedbecause it has been transformed. The method allows providing finer andmore precise information in order to constrain the storage behaviourmodel. The method, by allowing better monitoring the spatial evolutionof the fluid contacts upon injection, allows confirmation or, on thecontrary, to invalidate the flow model and, consequently, to follow moredistinctly the CO₂ front stored in the subsoil.

This allows better management of CO₂ injection and of the entireindustrial process. For example, it can lead to different storage siteexploitation conditions: location of the new injection well, injectionpressures modification, use of additives, etc.

It can also be noted that the monitoring method according to theinvention does not require drilling new wells in the storage zone, whichcontributes to considerably reducing leak risks. Well data are indeednot essential and, if required for inversion, it is possible and evenadvisable to drill far away from the storage zone so as to prevent leakproblems.

1-7. (canceled)
 8. A method of monitoring a CO₂ geological storage site,from a first set of seismic data imaging a subsoil zone which areacquired before a CO₂ injection in an underground formation of the zone,and from a second set of seismic data imaging the subsoil zone which areacquired after CO₂ injection, to locate a free CO₂ plume formed afterthe injection comprising: constructing by using stratigraphic inversionfor each seismic data set a P-wave seismic impedance cube and an S-waveseismic impedance cube; constructing a density variation cube from theP-wave and S-wave seismic impedance cubes before and after CO₂ injectionwherein the density variation cube discretizes the subsoil zone into aset of cells; constructing an incompressibility modulus variation cubefrom the P-wave and S-wave seismic impedance cubes before and after CO₂injection wherein the incompressibility modulus variation cubediscretizes the subsoil zone into a set of cells; and locating the freeCO₂ plume in the zone by identifying cells where the density variationis negative and the incompressibility modulus variation is negative andhas an absolute value greater than a positive threshold.
 9. A method asclaimed in claim 8, wherein a volume of the free CO₂ plume is determinedby: constructing a cube of CO₂ saturation in the formation from thedensity variation cube; and determining the volume of the free CO₂ plumeby summing all values of the CO₂ saturation cube and weighting the sumby a mean porosity of the formation and by a volume occupied by anelementary cell of the saturation cube.
 10. A method as claimed in claim9, wherein a mass of the free CO₂ plume is also determined bymultiplying the volume of the free CO₂ plume by a mean density of theCO₂.
 11. A method as claimed in claim 9, wherein the CO₂ saturation cubeis constructed by: determining an irreducible brine saturation S_(wi)for the formation; determining a maximum absolute value for the densityvariation cube and calculating a ratio by dividing the maximum absolutevalue by 1−S_(wi); constructing a CO₂ saturation cube by multiplyingeach value of the density variation cube by the ratio; and convertingthe saturation cube to depth by performing a time-depth conversion. 12.A method as claimed in claim 10, wherein the CO₂ saturation cube isconstructed by: determining an irreducible brine saturation S_(wi) forthe formation; determining a maximum absolute value for the densityvariation cube and calculating a ratio by dividing the maximum absolutevalue by 1−S_(wi); constructing a CO₂ saturation cube by multiplyingeach value of the density variation cube by the ratio; and convertingthe saturation cube to depth by performing a time-depth conversion. 13.A method as claimed in claim 9, wherein integrity of the storage site ischecked by detecting a CO₂ leak by analysis of the location of the freeCO₂ plume and stopping the detected leak.
 14. A method as claimed inclaim 10, wherein integrity of the storage site is checked by detectinga CO₂ leak by analysis of the location of the free CO₂ plume andstopping the detected leak.
 15. A method as claimed in claim 11, whereinintegrity of the storage site is checked by detecting a CO₂ leak byanalysis of the location of the free CO₂ plume and stopping the detectedleak.
 16. A method as claimed in claim 12, wherein integrity of thestorage site is checked by detecting a CO₂ leak by analysis of thelocation of the free CO₂ plume and stopping the detected leak.
 17. Amethod as claimed in claim 9, wherein exploration of the storage site ismodified after detecting a transformation of free CO₂ by a comparison ofa volume of free CO₂ with a volume of CO² that has been injected.
 18. Amethod as claimed in claim 10, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a volume of free CO₂ with a volume of CO² that has been injected. 19.A method as claimed in claim 11, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a volume of free CO₂ with a volume of CO² that has been injected. 20.A method as claimed in claim 12, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a volume of free CO₂ with a volume of CO² that has been injected. 21.A method as claimed in claim 13, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a volume of free CO₂ with a volume of CO² that has been injected. 22.A method as claimed in claim 14, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a volume of free CO₂ with a volume of CO² that has been injected. 23.A method as claimed in claim 15, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a volume of free CO₂ with a volume of CO² that has been injected. 24.A method as claimed in claim 16, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a volume of free CO₂ with a volume of CO² that has been injected. 25.A method as claimed in claim 9, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected. 26.A method as claimed in claim 10, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected. 27.A method as claimed in claim 11, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected. 28.A method as claimed in claim 12, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected. 29.A method as claimed in claim 13, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected. 30.A method as claimed in claim 14, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected. 31.A method as claimed in claim 15, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected. 32.A method as claimed in claim 16, wherein exploration of the storage siteis modified after detecting a transformation of free CO₂ by a comparisonof a mass of the free CO₂ with a mass of CO₂ that has been injected.